A senior detective who investigated child abuse allegations against Labour politician Greville Janner has revealed he was ordered to drop the case 'from the very top' – despite uncovering compelling evidence to charge him.

Breaking a 24-year silence over the scandal, former Detective Inspector Kelvyn Ashby told The Mail on Sunday that during an investigation lasting several months in 1991, he found vital clues that backed up claims that Janner had molested a teenage boy at his marital home and a hotel.

Last night the retired policeman spoke of his anger after being ordered not to arrest Janner because he was an MP.

Mr Ashby said: 'I felt we had done a good job. I felt we had enough to arrest him but we didn't because he was an MP. I think we should have done. I was gutted that we didn't.'

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A senior detective has revealed he was told to drop a sex abuse investigation against Lord Janner (pictured in 1972 with children who were not linked with abuse claims)

Detective Inspector Kelvyn Ashby (pictured) said he found vital clues that backed up claims that Janner had molested a teenage boy at his marital home and a hotel

And he revealed he was left sickened when Janner sent him a Christmas card thanking him for the way he was treated and inviting him to dinner at the Commons.

As the cover-up scandal deepens over Janner – now deemed too ill to stand trial despite evidence he abused nine boys over three decades – The Mail On Sunday can also reveal:

The head of children's charity the NSPCC has demanded the Director of Public Prosecutions explain her decision not to let the case against Janner be heard in court

More than 40 politicians have signed a letter condemning the way Janner has been allowed to escape justice

Home Secretary Theresa May told this newspaper that the Janner case could deter other victims of abuse coming forward

Janner went on more than 20 working trips abroad in the years after he was reportedly diagnosed with Alzheimer's

Janner was a well-known Leicester MP, barrister and author when in 1991 he was suddenly publicly accused of child abuse at the trial of a notorious paedophile.

Predatory children's home boss Frank Beck claimed he had tried to stop a boy in his care from visiting Janner, and sensationally called the alleged victim as a defence witness.

By then aged 30, the alleged victim told jurors he was only 13 when Janner had befriended him. He claimed the politician forced him into sex at a hotel, at his London home and on a tour of Scotland.

No charges were brought against Janner and he was backed by fellow MPs after telling the Commons he had been framed by Beck and his accuser.

Prosecutor Alison Saunders (right) who ruled that Lord Janner (pictured in 1996) would not face trial over alleged child sex abuse trained at the same legal firm where he was a QC

But today it can be told for the first time that police had been desperate to see Janner brought to justice after discovering evidence that corroborated the alleged victim's account.

We can reveal that officers went to the North London home where the boy was allegedly abused by Janner and found that it matched his description exactly, with an en suite bathroom off the master bedroom.

They also proved that Janner had stayed at the hotel in Scotland where he was said to have taken the boy.

SHAMEFUL HISTORY OF ALLEGATIONS AND INQUIRIES

1974 Greville Janner, Labour MP and QC, alleged to have begun two years of abuse with teenage boy living at a Leicester children’s home.

November 1991 Claims against Janner first made public at trial of paedophile children’s home boss Frank Beck, with the alleged victim giving detailed evidence against him.

December 1991 Janner defends himself in House of Commons and is supported by fellow MPs after police decide not to take action against him.

2002 Leicestershire Police launch Operation Magnolia into historic abuse at local children’s home and again investigate Janner, but take no action.

2006 Operation Dauntless looks at allegations of child abuse in 1981 by three men – two who were by then dead, and Janner.

December 2007 Local CPS lawyers decide not to charge Janner over the latest allegations – and breach normal procedures by not telling bosses in London since the case involved an MP.

2009 Janner diagnosed with Alzheimer’s, according to the CPS, but continues to speak and vote in the House of Lords, as well as going on 20 working trips abroad, in the next few years.

December 2013 Janner’s home in North London searched as police begin a new investigation, Operation Enamel, into historic sex abuse claims.

March 2014 Officers search Janner’s office in the House of Lords.

April 9, 2015 House of Lords receives letter, purportedly from Janner, saying he wants to continue his leave of absence, which started in October 2014.

April 16, 2015 CPS announce they have evidence to charge Janner with 22 child sex offences against nine victims, but will not do so because he has severe dementia.

Speaking in detail for the first time about the case last night Mr Ashby, now 65, said: 'He gave us an account of Janner's house, how many rooms it had and the layout of the furniture. When we visited, Janner had long since moved but the house was exactly as the alleged victim said it was. I was in no doubt he had been in that house.

'We looked at the Scottish tour and believed him, we established Janner stayed in those hotels but could not prove the boy was with him. But we had the note, his testimony, and the fact a boy had been taken out of care to stay in London with an MP.

'It meant we had enough in my eyes.' Mr Ashby – at the time a Detective Inspector and the Senior Investigating Officer in the Beck case – was working with a Detective Sergeant called Mick Creedon, now Chief Constable of Derbyshire Police.

Mr Ashby said: 'Mick and I believed an arrestable offence had been committed and that once we arrested him we could search his house and see if there was any material that helped corroborate what the alleged victim said.

'Someone higher-up told us that we couldn't just arrest an MP and it went no further. We were told that by someone senior, who I can't name, but the order had to have come from the very top. I'm sure my bosses' hands were tied.

'I was extremely frustrated. We might have found more in his house, or maybe more victims would have come forward. Janner should have been arrested. He was treated differently because he was an MP.' Janner was questioned at a police station in Leicester but refused to answer questions.

Mr Ashby, who left the police in 2002, went on: 'The bit that really got me was that I later got a Christmas card in the post from Greville Janner.

'The card was an official House of Commons Christmas card and was handwritten. It said something like 'I was very pleased with the way you treated me' and invited me and my wife for a dinner at the House of Commons. I couldn't believe it.

'My wife was disgusted, as was I. Needless to say we never replied and never went. I'm only speaking now because the police investigation has stalled. Also, I have to think about the victims and I feel they have been let down.'

A decade later in 2002, Leicestershire Police began another investigation against Janner but no charges were brought.

Then after a third probe in 2006, by which time Janner had been given a peerage by Tony Blair, detectives passed a file to the Crown Prosecution Service but local officials chose not to charge him.

In 2013 Leicestershire Police began yet another investigation against Janner and got as far as searching his London home and House of Lords office, while more than 20 victims gave statements against him.

Ten days ago Director of Public Prosecutions Alison Saunders announced there was enough evidence to charge Janner with 22 offences against nine alleged victims – but he could not be prosecuted because he has severe dementia.

The CPS claimed Janner was diagnosed with Alzheimer's in 2009. But analysis of his activities since then shows he went on at least 20 foreign trips, including to Israel and America as part of his work fighting anti-semitism and bringing Nazis to justice. He was still chairman of an all-party Parliamentary group two years ago.

Last night Leicestershire Police declined to comment on Mr Ashby's revelations, but confirmed it is now looking again into the earlier cases.

Janner's family have issued a statement insisting he was 'entirely innocent of any wrongdoing'.

NSPCC chief piles pressure on beleaguered DPP to go

The head of Britain's top children's charity last night condemned the Director of Public Prosecutions (DPP) for her bungling of the Lord Janner child abuse scandal – putting her under greater pressure to quit.

Peter Wanless, the chief executive of the NSPCC, demanded that Alison Saunders explain her controversial decision to keep the sickening allegations against the Labour peer out of court. And he warned her poor handling of the case will deter victims coming forward in the future – and could heighten fears of an Establishment cover-up.

Mr Wanless's accusations, in a letter seen by The Mail on Sunday, will put the beleaguered DPP under greater pressure to step down or reconsider her decision to spare Lord Janner prosecution on the grounds that he has dementia.

In his letter, sent on Thursday, NSPCC chief and key Government adviser Mr Wanless told the DPP he was not challenging the medical evidence that Lord Janner has Alzheimer's so is unfit to stand trial.

Peter Wanless (pictured) the chief executive of the NSPCC, demanded that Alison Saunders explain her controversial decision to keep the sickening allegations against the Labour peer out of court

But he demanded answers as to why law chief Mrs Saunders did not choose a well-established procedure often used when suspects lack mental capacity, known as a 'trial of facts', in which jurors hear evidence but do not find guilt and no punishment is imposed by the judge.

Mr Wanless said justice should have been done – as the CPS admitted last week it should have charged him after three earlier investigations by Leicestershire Police.

He wrote: 'Given the exceptional historical mistakes in this matter, I would like to understand why you did not deem it in the public interest to have a trial of facts, given this legal mechanism exists to enable the alleged victims to present their evidence in court and have a decision made as to whether Lord Janner carried out the alleged acts.

Given the exceptional historical mistakes in this matter, I would like to understand why you did not deem it in the public interest to have a trial of facts, given this legal mechanism exists to enable the alleged victims to present their evidence in court and have a decision made as to whether Lord Janner carried out the alleged acts

Peter Wanless, NSPCC Chief Executive

'The decision by the CPS enables the disparity in the public arena between the position of the alleged victims and those of Lord Janner's family to exist in perpetuity.'

He went on: 'With victims of child sexual abuse, it is the very fact of being able to give evidence in court and have a decision made on the allegations which is so crucial.

'The courage required to come forward when you are or have been a victim of child sexual abuse should not be underestimated.

'We are concerned as to the unintentional consequences the situation could have on encouraging other victims of child sexual abuse to come forward, particularly if the accused sits in a position of influence or power.'

Mr Wanless's letter will carry weight as he is a former senior civil servant who last year was called upon by the Home Office to search its archives for the notorious 'Dickens dossier' on alleged VIP paedophiles.

Mrs Saunders, however, believes it would be wrong to hold a trial of facts, because they are usually used when the suspect poses a danger to the public and a judge needs to impose an order to ensure they receive hospital treatment.

She told the BBC last week: 'The medical evidence was very clear in this case that there was no ongoing risk.'

Mr Wanless still believes that Lord Janner's accusers need to be satisfied that justice has been done.

Mrs Saunders suffered another knock last night after a Survation/MoS poll said calls for her to resign are supported by 44 per of the public with 27 in favour of her staying on.